XMonad is not very usable on OS X, as most apps are Cocoa or Carbon apps, meaning they don't run under Apple's X11.app and thus not within XMonad.

You could, however, probably install GTK+, WxWidgets, Qt... and rebuild many apps like Firefox, Emacs... to use X11.

Potential Configuration Problems

You may have problems with configuring XMonad under OSX - if you see the "error detected while loading xmonad configuration" error message but with no error output, it could be because the PATH inherited by XMonad doesn't include ghc (eg by default it won't include /usr/local/bin).